This invention is particularly applicable to shipboard re-liquefaction of boil-off natural gas from LNG carriers, where simplicity, weight, energy consumption, cost, and maintenance must strike an economic balance.
Such systems have typically incorporated a refrigeration cycle, composed of a working fluid such as nitrogen gas in multi-stage compression and one or two turboexpanders which may drive compressors; and the boiloff gas is typically compressed in two stages. Such prior art is shown in existing patents: WO 98/43029 A1 (Oct. 1, 1998), WO 2005/057761 A1 (May 26, 2005), WO 2005/071333 A1 Aug. 4, 2005, each issued to Rummelhoff; and U.S. Pat. No. 6,449,983 B2 (Sep. 17, 2002) and U.S. Pat. No. 6,530,241 B2 (Mar. 11, 2003), each issued to Pozivil; and has also been prominently displayed in publications and web sites. The designs in the prior art include turboexpansion of the refrigerant gas through wide pressure and temperature ranges, considered essential for process efficiency under the selected overall plant design, leading to compression of the refrigerant gas in multistage compressors of increased weight and complexity. None of these patents (and other published material) has openly considered the viability of a single stage of refrigerant compression, though shipboard liquefaction of boiloff gas has been a topic of serious investigation. Hence, the advantages of single-stage compression of a refrigerant gas in a main compressor have not been obvious to practitioners with skill in the specific technology.
Since these installations are considered primarily (but not exclusively) aboard ship, size and weight, and the number of pieces of equipment, especially machinery, take on great importance. Additionally, requirements for unbroken on-stream time may necessitate full duplication of all rotating equipment, effectively doubling the savings which accrue from a reduction in component machinery and complexity.
In view of the compound requirements for achieving efficient reliquefaction and reducing the number of components, including their weights and complexity, it would be advantageous to develop a process which achieves both ends.
It has been determined that under certain design configurations, a refrigeration cycle requiring a main single-stage compressor for the refrigerant, can have high thermodynamic efficiency (low specific power); and have the aforementioned benefits of reductions in component rotating equipment.
The current invention breaks the state-of-the-art barrier to an efficient refrigeration cycle based on a low compression ratio for the refrigerant gas, and enables employment of a single-stage main compressor for the refrigerant gas. The current system offers attractive alternatives to other proposed and constructed systems.
This invention achieves the objectives of net capital cost and overall weight reduction by reducing the compression of nitrogen in a main compressor to one centrifugal stage, saving a large investment over a main compressor of multiple stages and its coolers. Further compression may take place in compressors which are shaft-connected to turboexpanders.
Another aspect of this invention is that the refrigeration cycle is so designed as to efficiently achieve boiloff gas condensation while utilizing only one turboexpander, while maintaining a low compression ratio on the single-stage refrigerant compressor.
This invention relates to a process and equipment configuration to liquefy natural gas boiloff, wherein gas machinery for the refrigeration cycle is composed of a single-stage main compressor and one or two turboexpanders, which may drive compressors.
Additional improvements may include, all or individually, a single-stage boiloff gas compressor; an inserted heat exchanger to enable compression of the boiloff gas from an ambient temperature condition; and throttling a small refrigerant sidestream at low temperature in order cover the complete cooling range, while maintaining a low compression ratio on the single-stage main cycle compressor without an increase in energy consumption. This is especially effective when the condensed boiloff gas is brought to a subcooled condition.